This invention relates to the use of mixed alkyl esters made by reacting two or more of certain monohydric alcohols with interpolymers which contain units derived from (i) .alpha.,.beta.-unsaturated dicarboxylic acids, or derivatives thereof and (ii) vinyl aromatic monomers having up to 12 carbon atoms in crude oils. Minor amounts of the mixed alkyl esters are useful for modifying the fluidity and flow characteristics of crude oils, and more particularly, for improving the pipeline pumpability of crude oils.
Crude oils are transported over long distances through pipelines, and the pumpability of the crude oils through the pipelines is an important consideration. Most crude oils are characterized by their high natural pour points thereby requiring the addition of pour point depressants and fluidity improves as an aid to pipeline pumpability. Various materials have been suggested in the prior art as fluidity improvers in liquid hydrocarbons which are highly desirable and useful. However, many of the known fluidity improvers have not proved entirely satisfactory in improving the fluidity characteristics of a wide variety of liquid hydrocarbons. Some fluidity improvers have been found to be effective in certain types of oils while exhibiting more limited improvement in other types of oils. More specifically, some of the pour point depressants which have heretofore been used to control the pour point of distillate fuels and lubricants have been found to be either ineffective or to show only slight improvement in lowering the pour point of crude oils.
In addition to lowering the pour point of crude oils, it also is important to modify other properties of crude oils in order to improve the pipeline pumpability of the crudes. For example, it is desirable to lower the plastic viscosity and the yield value of the crude oils which are to be transported through pipelines. The yield value can be defined as the minimum force required to "move" the crude oil from a static position at a given temperature. Thus, yield value measurements assist in predicting the ease of re-starting a shut-down pipeline.
As mentioned above, some fluidity modifiers have proved effective in certain types of oils while exhibiting more limited improvement in other types. For example, in U.S. Pat. No. 3,536,461, pour point depressants comprising esters of a styrene and maleic anhydride polymer and longchain fatty alkanols of 20 to 22 carbon atoms are reported to be effective to lower the pour point of both raw and hydrotreated shale oil. However, the corresponding ester derived from an alkanol containing 18 carbon atoms is effective in lowering the pour point only of shale oil which has been hydrotreated.
The use of esters of styrene-maleic anhydride copolymers in lowering the pour point of hydrocarbon oils including crude oils and residual oils is described also in U.S. Pat. No. 3,574,575. The patentees report that there is no significant improvement in the fluidity characteristics of the crude oils tested (demonstrated by pour point data) when the esters are derived from alkanols which contain less than 20 carbon atoms in the alkyl portion. In this patent, the esters containing at least 20 carbon atoms were compared to esters containing 18 carbon atoms, namely, the di-1-octadecyl ester of styrene-maleic anhydride copolymer.
An improvement in the process for producing waxy crude oil from wells is described in U.S. Pat. No. 3,879,177 wherein an agent is added to the crude which is prepared by esterifying a copolymer of maleic anhydride and vinyl methyl ether with docosanol or a mixture of alcohols containing 18 to 24 carbon atoms. The improved process is reported to effectively inhibit the crystallization of wax from a waxy crude oil.